For example, Publication of Unexamined Japanese Patent Application 5-128400 (JP-A-5-128400) discloses a conventional art. In the art of JP-A-5-128400, when a vehicle travels on a road provided with a roadside beacon, the roadside beacon transmits information to the vehicle. The transmitted information is related to a position of a traffic light currently being ahead of the vehicle in a traveling direction and parameters of a lighting color change of the traffic light. The parameters may include a cycle, a split, and an offset of the lighting color change. The vehicle receives the transmitted information on the parameters and determines a time period, in which a blue light of the traffic light is lighting, according to the received parametric information. In addition, the vehicle calculates a speed range by which the vehicle can pass the blue light of the traffic light and indicates the calculated speed range on a display device.
According to the art disclosed in JP-A-5-128400, a driver of the vehicle drives the vehicle so as to maintain the speed of the vehicle within the speed range indicated on the display device, and thereby reducing a number of start and stop of the vehicle. As a result, fuel consumption of the vehicle can be reduced. However, when a lighting time of a red light of a traffic light being ahead of the vehicle is long enough, the speed range, by which the vehicle can pass a blue light of the traffic light, may be calculated to be a low-speed range. In this case, a driver of the vehicle has to drive the vehicle to maintain the low-speed range. Consequently, the driver may feel uncomfortable while driving the vehicle to maintain the low-speed range.